1. Field of the Invention
The present invention relates to the field of code verification, and more particularly relates to apparatus for verifying the accuracy of codes inserted in the vertical intervals of video signals.
2. Background Art
The optical video disc is a relativly recent technological development which provides expanded flexibility in the programming and playback of video program material, as compared with magnetic tape. This flexibility is typically implemented through the use of special codes which are inserted in the vertical intervals of the video signals which are recorded on the disc.
For example, constant angular velocity ("CAV") video discs have particular versatility by virtue of the fact that the vertical intervals of the fields of video recorded on the disc line up in discrete radial directions. The most typical CAV format provides two fields of video in each spiral track, such that the vertical intervals of Field 1 of all frames line up in one radial direction and those of Field 2 in a second radial direction 180.degree. opposite the first direction. The CAV disc is played by rotating the disc at a constant angular velocity and translating an imaged beam of light across the disc to follow the spiral pattern of the tracks of video information on the disc.
Because of the symmetrical arrangement of recorded video on CAV discs, special effects, such as slow motion and stop motion playback, are rendered relatively easy to implement as compared with other video recording and playback media, such as tape. For example, stop motion playback of a particular frame of video is effected by rotating the disc at its normal rate of rotation, scanning the imaged beam of light along the single track containing the signals for the frame being played, and causing the light beam to jump back to the beginning of that track once every rotation of the disc. Due to the symmetrical arrangement of the video fields recorded on the disc, horizontal and vertical synchronization are maintained smoothly during such stop motion playback mode without the need of special resynchronization circuitry.
Searching for specific frames of video can be effected quite rapidly with video discs, whether they are of the CAV type or of the constant linear velocity ("CLV") type which are played at a rotation rate which maintains a constant linear velocity of the reading spot of laser light with respect to the track being read. For example, if the spot of laser light is to be moved from the beginning of the program material on a disc in a search for a frame relatively near the end of the program material, the spot is merely moved radially to the vicinity of the frame being searched and then tracks are selectively scanned according to an algorithm which rapidly locates the frame being searched. Sweeping a spot of laser light across an optical disc can be done far more rapidly than advancing conventional video tape through such a span of program material, hence the particular usefulness of the optical video disc in such a mode.
In order to permit the implementation of these special playback modes it has been found desireable to insert special codes during the vertical blanking interval of the program video which is recorded on the disc. These special codes serve to identify those frames where jumpback in a special mode, such as stop motion, is permissible to provide a jitter-free frame of displayed video in that mode, and serve to identify frames and other selected subdivisions of the program material for these special modes.
The special codes are inserted into the video in accordance with a procedure which represents a set of rules related to the structure of the video content of the signals, for example, to the pattern of repetition of identical fields of video.
The accurate placement of these coded signals in accordance with the aforementioned procedure is important to ensure that the desired special playback mode capability is fully provided for an entire segment of such video. However, there have heretofore been no apparatus which automatically determine if the specially coded signals have been thus accurately placed on a given segment of recorded video. Checking an entire segment of such video, for example a one or two hour segment of video, has therefore been done by manual inspection on a field-by-field basis of the segment of video using video signal monitoring equipment. Such a practice is time consuming and costly.
Accordingly, there is a need for an apparatus which permits the automatic inspection of the video signal content of a recorded segment of program video having the aforementioned special codes inserted therein, to determine if those codes have been properly inserted. The present invention satisfies this need.